Woot via Amazon offers Prime members the refurbished Arris Surfboard DOCSIS 3.0 Cable Modem, model no. SB6121, for $29.99 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $8.77.) That's $38 under what you'd pay for a new one. It features support for IPv4 and IPv6 and enhanced network management.
A 90-day warranty applies, but it's unclear who backs it.